if you receive the coin via regular CD/DVD envelope mail shipping and have me omit the presentation box, 99% of the time it slips through and nobody notices it. In fact, I have never heard of anyone ever having customs care about anything that fit in a CD/DVD envelope. It is always express mail that gets their attention, and sometimes boxes, that are a magnet for being charged VAT by customs. The way I ship it, by the way, it's hard to even tell it's a silver coin (when you see it you'll know what I mean, but I don't want to spoil it).
I've also found this on reddit about a guy who sells UK casascius coins who has also been hit: http://www.reddit.com/r/Bitcoin/comments/17xql4/first_dedicated_uk_based_casascius_physical/
What would be great is if someone who is taking a trip to/from the US/UK brought some through in their hand luggage, I think this way they are unlikely to be hit